In the plating of dielectric substrates by chemical (electroless) plating it is well known that suitable catalytic pretreatment is a prerequisite for effective electroless metal deposition. Such practices are well known and accepted in the art.
In examining the prior art for catalytic pretreatment it appears that while different procedures have been used, the incorporation of precious metals (e.g., palladium containing solutions) was common to all procedures. One catalytic system of particular interest is the two step process as disclosed in U.S. Pat. No. 3,011,920. In the process disclosed, a colloidal solution composed of tin(II) and precious metal salts, generally with hydrochloric acid, is used. The effective catalyst is proposed to be a colloid of an elemental precious metal (e.g., palladium) stabilized by the excess stannous chloride present in the medium. While the system disclosed in U.S. Pat. No. 3,011,920 has been quite popular in commercial practices, rising costs of precious metal, instabilities due to air oxidation, and miscellaneous product reliability problems have led to the quest for new systems in which the use of precious metals, tin, as well as hydrochloric acid would be completely eliminated.
In meeting this objective it was found, as described in U.S. Pat. No. 3,958,048, 3,993,799, and 4,087,586 that colloidal systems based upon non-precious metals could constitute the basis for new commercial plating processes. More specifically, it was found and reported that colloidal compositions of non-precious metals (preferably selected from the group of copper, cobalt, iron, nickel and manganese) may be used in the direct replacement of the tin/palladium colloid followed by a treatment (which may be optional) in a suitable reducing (activating) medium. In the latter treatment a precursor derived from the colloidal dispersion constitutes the catalytic sites useful in the initiation of plating. In the reducing medium, reduction of the ionic portion of adduct derived through the adsorption from the colloidal medium takes place or surface activation, resulting in active nucleating sites capable of initiation of the electroless process.
In reviewing the teaching disclosed in the aforementioned issued patents which are included herein by reference, it is recognized that many of the inherent disadvantages associated with the palladium based catlysts are eliminated. It is further recognized that based upon practices in this art it is further essential that any catalytic system should maintain its properties especially with storage (e.g., several months) and shipment under conditions of substantial temperature fluctuation. It is thus highly desirable to have a medium in which good colloidal stability would be maintained, and which at the same time has sufficient catalytic activity to be used in the plating process. I have observed that as one increases stability, activity is decreased thereby making it difficult to meet both requirements in a single preparation step. The activity of the colloid apparently is diminished by the surrounding of the colloid nucleus with the colloid stabilizer(s) used.
For example, I have observed that successful synthesis of active plating colloids has generally shown a limited stability (for long term storage purposes) due to coagulation which takes place leading to precipitation, with, of course, change in particle size and distribution during the coagulation process. In addition, I have noted that highly stable colloidal dispersions have shown limited catalytic activity when used in accordance with U.S. Pat. No. 3,993,799 with a moderate concentration of reducing medium or activating medium or the omission of any secondary step. Similar trends were also noted in U.S. Pat. No. 3,958,048 on the interrelationship between reactivity and stability. In fact, in U.S. Pat. No. 3,958,048 some of the illustrated examples lost their colloidal character and became true solutions within 24 hours.
It is thus highly desirable to provide highly stable colloidal dispersions and at the same time provide a simple way by which the stable colloids may be transformed via a modifier into an active catalytic state (form) useful in electroless plating processes or any other processes utilizing colloids. Furthermore, in said transformation, the metallic portion of the colloid is modified without the apparent change in the formal oxidation state. It is further desirable to obtain dispersions with very fine particle size distributions. Small size dispersions are particularly useful in adsorption processes and catalysis, and in particular tend to exhibit good stability.
While not wishing to be repetitious, the following are included herein by reference: U.S. Pat. No. 3,011,920, 3,993,799, 3,958,048, 3,993,491, 3,993,801, 4,087,586 and 4,048,354.